This invention relates to a drive circuit for a brushless motor.
Such a circuit using a PWM circuit for saving power has been known in the art. Its circuit configuration is shown in FIG. 5. In FIG. 5, 41 denotes a resistor for detecting the value of a current supplied to each of phase coils U, V, and W; 42 denotes a comparator for comparing a control input for controlling speed with the supply current value; 43 denotes a power supply for level regulation; 44 denotes a chopping wave generator circuit; and 45 denotes a comparator. The chopping wave generator circuit 44 and the comparator 45 generate a pulse depending on the output of the comparator 42. Q2-Q7 denote transistors for driving the phase coils U, V, and W; and Q2A, Q4A, and Q6A are transistors for driving the transistors Q2, Q4, and Q6, respectively.
1 denotes an current switching circuit for supplying a current to predetermined phase coils whose output is generated by a starter circuit 2, an oscillator 3, and a ring counter 4 when starting the motor and is generated by the back electromotive voltages of the phase coils U, V, and W during the operation which are detected by OP amplifiers 5, 6, and 7. 8 denotes a start/back electromotive voltage detect switching circuit for determining output to the outside.
46 denotes a phase switching PWM pulse synthesizing circuit, which selects transistors to be turned on according to the output of the current switching circuit 1 and performs PWM control on the selected transistors according to the output pulse of the comparator 45. In the circuit shown in FIG. 5, the transistor Q2, Q4, or Q6 connected to the supply voltage VDD is completely turned on at starting, while only the transistor Q3, Q5, or Q7 connected to ground is PWM controlled by the control pulse during driving. 47 denotes a snubber circuit for eliminating noise which is generated when the transistors are switched.
The circuit described above is a drive circuit for a three-phase brushless motor, and uses the PWM circuit for saving power. This circuit compares the control input for speed control with the detected current value, and further compares the result of that comparison with the chopping wave to generate a pulse waveform and to switch the transistors to be driven.
Since this circuit performs switching relating to the PWM control for each phase, it requires a snubber circuit using three capacitors having large capacitance. Thus, the circuit has a problem that it requires a large packaging area and is expensive.
In addition, since the switching noise cannot be completely eliminated, electromagnetic noise produced by the motor coil may enter a signal detecting head and deteriorate the S/N ratio when the circuit is used for driving a spindle motor of a hard disk drive. Moreover, it cannot use a soft switching circuit which slowly switches the phase current to operate the driving stage. Therefore, there has been a problem due to rapid phase current switching that electromagnetic noise of the motor may enter the head and mechanical noise may occur.
The configuration of another conventional circuit is shown in FIG. 6. In FIG. 6, the same reference numerals as those used in FIG. 5 denote the same components. 10 denotes a soft switching waveform shaping circuit for making the rising or falling of current gradual, and 11 denotes a three-differential amplifier which selects transistors to be driven according to the output of the current switching circuit 1 inputted through the soft switching waveform shaping circuit 10, and controls the selected transistors. This three-differential amplifier controls the base current of the driven transistor according to the output of a comparator 12 which compares the speed control signal with the detected current value. In this case, the rising and falling of this base current are made gradual by the soft switching waveform shaping circuit 10. Moreover, the transistor Q2, Q4, or Q6 connected to the supply voltage VDD is completely turned on when it is driven, and only the transistor Q3, Q5, or Q7 connected to ground controls the collector current according to the base current inputted when it is driven.
The circuit such as shown in FIG. 6 has been popular (for example, it may be Panasonic AN 8221 FB manufactured by Matsushita Electronic Industries, Inc.), and widely used for driving a spindle motor of a hard disk drive.
In the circuit configured as above, the current flowing through each of the phase coils U, V, and W of the motor does not rapidly change as compared with the PWM control. Thus, it is possible to reduce the electromagnetic noise of the motor which would enter the head. In addition, the slow or gradual current switching allows it to reduce the motor vibration and hence the mechanical noise. Furthermore, since there is no rapid switching, the capacitors of the snubber circuit may be of small capacitance and the snubber circuit itself may even be eliminated to reduce the packaging area.
As described above, the conventional circuit shown in FIG. 6 has reduced electrical and mechanical noises and is functionally superior. In this circuit, however, since the transistors Q3, Q5 and Q7 connected to ground control the collector current according to the base current in a state where almost all of the differential voltage between the supply voltage VDD and the voltage applied to the phase coil is applied when they are driven, there is a problem that these transistors waste power.